Field devices for capacitive fill level measurement have been known for many years. In these devices, a probe extending into the container forms a capacitor with the container wall. The capacitance of the capacitor created in this way depends on the fill level and the dielectric constant of the medium to be measured.
A possibility for measuring this capacitance is the so-called apparent current measurement method. In this, a conventional rectifying circuit is used to measure the amount of the alternating current flowing through the capacitor formed from probe, medium and container wall for given frequency and voltage. The apparent current is, however, not only dependent on capacitance, but also on the conductivity of the medium to be measured, a fact which leads, in practice, to inaccuracies in the measurement, since the conductivity depends, especially in the case of bulk goods, on different factors, such as e.g. temperature and humidity.
A method for suppressing the influence of this parallel conductivity is measurement at relatively high frequencies. Since the part (reactive current) of the apparent current flowing through the capacitance increases in proportion to the frequency, while the part caused by the conductivity remains constant, the capacitive part predominates practically always at high frequencies. However, experience shows that measurement at high frequencies (>100 kHz) leads to difficulties with long probes in the form of large, parasitic inductances.
Another method is to measure not the apparent current, but rather the reactive current at a phase difference of 90° between current and voltage, this corresponding to a pure capacitance measurement. This can be done with the help of a synchronous rectifying circuit. With this method, however, there is the disadvantage that e.g. in media with small dielectric constant and large conductivity, in which measurements using apparent current measurement are possible without problem, difficulties arise because reactive current practically disappears. Moreover, such conventional synchronous rectifier circuits have proven to be sensitive with respect to electromagnetic interferences.